Permeable surfaces

With most residential work, be it a new home or a remodel, the design of the exterior “hardscape” is just as important as the home itself. These areas can include an entry sidewalk and its sequence of experiences leading up to the front door, a deck or a private courtyard integral to the home, or something as utilitarian as a driveway to the garage. In a nutshell, we’re talking about any hard surface areas exposed to the elements. Rather than being afterthoughts or secondary design elements, these features are central to the overall planning of the site. Traditionally, these hard surfaces prevented rain water from reaching the ground below and were considered impermeable by most building departments. But this is changing, and there are a variety of hard surface applications that allow for stable, flat surfaces, all while allowing rain water to pass through to the ground below.

These engineered materials are becoming increasingly important to architecture, construction and the permitting process. Most urban and suburban jurisdictions operate with land use codes that now limit the amount of lot coverage and impervious surface area on a parcel*. Doing so controls the amount of rainwater at the source, reduces runoff of storm water and improves water quality by filtering the pollutants within the soil itself. In the Pacific Northwest, for instance, the maximum allowable impervious surface area ranges from around 15 to 35 percent of the total parcel (depending on how the city or county has zoned the property). Once this limit is reached with the impervious surface area, no further development can occur on site. Because permeable surfaces are excluded from this calculation, they allow the addition of hardscapes without the penalty of impervious surface area—it allows the homeowner to have their cake and eat it too.

We work with a high frequency of challenging sites; sites which are usually governed by stringent zoning codes, and we typically need to be conscientious and careful of the allowable impervious surface area. This has necessitated a kit-of-parts over the years that include some reliable and cost-effective permeable surfaces. Today’s post breaks our findings down into three groups:

Because the technology here is ceramic-based, these systems are typically proprietary. We like the Aroura Klorostone product for its clean aesthetic, simple color options and versatility.

Permeability: The Klorostone is capable of infiltrating up to 2 inches of stormwater per minute without relying on mortar gaps. The exact permeability is difficult to determine with these products because each company has its own protected recipe.Compressive strength: 6,000 psiThickness: 2-3/8″Technology: Each individual paving unit is porous (as opposed to interlocking concrete that relies on aggregate gaps for infiltration)Application: Driveways, sidewalks, courtyards, patiosCost: $7 – $10/sf deliveredAvailable colors: 4Other considerations: The joints around paving units will provide an additional 5-15% of permeable area

We’re big fans of the Belgard Turfstone pavers because of the large paver size (24” x 16”); this size can be used for a continuous surface or it’s just wide enough to accommodate two tire strips for an inconspicuous driveway.

Permeability: The units preserve 40% of earth area and allow water to pass directly through to the earth below.Compressive strength: 8000 psiThickness: 3-1/8”Technology: Concrete and holes, it’s simple and straight-forward (just how we like it)Application: We like to use it for courtyards or understated drivewaysCost: ~$5/sfAvailable colors: GrayOther considerations: Fill the open cells with moss or grass for a softer visual (it also helps keep the site cooler)

*It’s worth noting that gravel is considered an impervious surface by many jurisdictions and its inclusion on a project will count against the impervious surface calculations. From the King County website:

“Packed gravel prevents or impedes the entry of water into the soil as compared to natural conditions. Scientific studies show that once gravel is compacted (from cars or heavy equipment, for example), the gravel acts like paved surfaces and surface water runs off it in greater quantities than compared to natural conditions. In addition, if cars or heavy equipment are traveling on these gravel surfaces, pollution such as dissolved minerals or residual petroleum are washed off into our waterways.”

There’s tons of products out there that help with the permeability of surfaces in our built-environment, share what you know and feel free to add to our list of favorite products.

16 Comments

You mention that gravel is typically not considered a porous material because when compacted it acts more like a paved surface. What about when it’s not allowed to compact? For example, when used in conjunction with open cell pavers as shown in the second photo from the top. Would the building department allow this assembly as permeable?

I’ve come across building departments that require the percentage of permeability for pavers before they’ll approve them as a permeable surface. Are companies like Aroura willing to clarify that information?

Here in Denver, the fire dept was amenable at first to the grass infilled pavers as a better more appealing fire lane surface, but then they found out that if the grass grew too well, they lost sight of their access roads. Ironic.
Not to mention we don’t get that much moisture here, so the only thing clients have found them good for locally is LEED points.

I’m still determined to specify that permeable asphalt though. That stuff would make shoveling the walks much less arduous if the snow is allowed to melt through it. Great post-thanks guys!

we actually prefer to use products like grasspave2 simply because with our weather here in Texas, those concrete pavers retain a lot of heat and they’ll fry the roots of anything that gets planted in the cells …

We’ve specified porous ashphalt in the past, but found that over time the voids become clogged with debris – which then needs to be vacuumed out (which is amusing to watch a driveway being vacuumed).
We now specify a pea-gravel system, because the proceedure can get pricey…

In reference to Dennis’ gravel response, I don’t think it’s the gravel being compacted but the soil underneath. Water still penetrates the 2″ – 3″ of gravel on top , but it then collects on the compacted subbase with nowhere to drain.

Aroura’s product is 100% porous surface area. The brick itself is your permeable surface so no deductions need to be considered when doing your calculations. (as is the case with spaced concrete pavers). I’ve installed Klorostone in a community garden for ADA access and in front of a high school gym entrance to control water pooling and ice. Both jobs weathered over the winter well.

I had always thought of pervious pavement as a gimmick. But I recently moved and my new polling place is at a ecopark type facility and when I went to vote it was raining. It was really nice to not have to dodge puddles. But I still worry about long term oil contamination. Seems like all that gunk will leech through the pavement. But, perhaps better than washing into special drain systems

I love the materials mentioned in this post (particularly the grass-crete pavers), but it’s worth mentioning that you may want to get a soils report prepared by a qualified geotechnical engineer first if you don’t know what you’re building on. With moderate to highly expansive material, using pervious pavement in close proximity to your building could cause some problems.

Very cool post, thanks for sharing this great product! Geez that would be great here in Texas! We get the ground so dry here in the summer, and large cracks take over the yards! I think this would be a great solution, and you could incorporate it into your sprinkler system design! I say genius!

Not a fan of the open cell pavers myself, but a local restaurant recently installed them for use in a parking lot. It’s not something you see often, so many folks drive in confused on where to park. I much prefer the compressed / porous designs and think they’re much easier to walk on.